Steels used for a liquefied natural gas (LNG) tank need to have fracture-resisting performance at an extremely low temperature of approximately −160° C. For example, so-called 9% Ni steel is used for the inside tank of the LNG tank. The 9% Ni steel is a steel that contains, by mass %, approximately 8.5% to 9.5% of Ni, has a structure mainly including tempered martensite, and is excellent in, particularly, low-temperature toughness (for example, Charpy impact-absorbed energy at −196° C.). With an increasing demand for natural gas in recent years, in order to satisfy an increase in the size of the LNG tank, there is a demand for additional improvement in the fracture resistance of the tank. As one of the fracture-resisting performances, various techniques to improve the toughness of the 9% Ni steel have been disclosed. For example, Patent Documents 1 to 3 disclose techniques in which temper embrittlement sensitivity is reduced by a two-phase region thermal treatment so as to improve the toughness. In addition, Patent Documents 4 to 6 disclose techniques in which Mo that can increase strength without increasing the temper embrittlement sensitivity is added so as to significantly improve the toughness. However, since the manufacturing costs increase in the methods of Patent Documents 1 to 6, it is difficult to use the methods at a low cost for the LNG tank which has a strong demand for fracture-resisting performance. Meanwhile, steel plates having a plate thickness of 4.5 mm to 80 mm are used as the 9% Ni steel for the LNG tanks. Among them, a steel plate having a plate thickness of 6 mm to 50 mm is mainly used.